[I] Enable parallel VACUUM (currently disabled via GPDB_14_MERGE_FIXME due to lock issues) [cloudberry]

["my-ship-it (via GitHub)" <gi...@apache.org>]

my-ship-it opened a new issue, #1858:
URL: https://github.com/apache/cloudberry/issues/1858

   ## Summary
   
   PostgreSQL 16's **parallel `VACUUM`** (parallel index vacuuming) is currently **disabled** in Cloudberry. `VACUUM (PARALLEL n)` is accepted syntactically, but at runtime the parallel option is silently downgraded to serial with a `WARNING`, on both the coordinator and every segment. The real parallel code path is compiled out.
   
   ## Where
   
   `src/backend/access/heap/vacuumlazy.c:3209`
   
   ```c
   /* GPDB_14_MERGE_FIXME: Don't support parallel vacuum now, we need to fix lock issues. */
   if (nworkers > 0 && vacrel->do_index_vacuuming && vacrel->nindexes > 1)
       ereport(WARNING,
               (errmsg("disabling parallel option of vacuum on \"%s\" --- cannot vacuum tables in parallel",
                       vacrel->relname)));
   #if 0
       else
           ...   /* the actual parallel_vacuum_init() / parallel path is #if 0'd out */
   ```
   
   So any table with more than one index that is asked to vacuum in parallel hits this guard, and the upstream parallel path below it is dead-coded via `#if 0`.
   
   ## Reproduction
   
   Environment: PostgreSQL 16.9 / Apache Cloudberry 3.0.0-devel (`main`, `c781604c5bad`), 3 primary segments.
   
   ```sql
   CREATE TABLE pv_demo(id int, a int, b int, c text) DISTRIBUTED BY (id);
   INSERT INTO pv_demo SELECT g,g,g,md5(g::text) FROM generate_series(1,1000000) g;
   CREATE INDEX ON pv_demo(a);
   CREATE INDEX ON pv_demo(b);
   CREATE INDEX ON pv_demo(c);
   DELETE FROM pv_demo WHERE id % 3 = 0;
   
   VACUUM (PARALLEL 3, VERBOSE) pv_demo;
   ```
   
   Output:
   
   ```
   WARNING:  disabling parallel option of vacuum on "pv_demo" --- cannot vacuum tables in parallel
   WARNING:  disabling parallel option of vacuum on "pv_demo" --- cannot vacuum tables in parallel  (seg0 ... )
   WARNING:  disabling parallel option of vacuum on "pv_demo" --- cannot vacuum tables in parallel  (seg1 ... )
   WARNING:  disabling parallel option of vacuum on "pv_demo" --- cannot vacuum tables in parallel  (seg2 ... )
   INFO:  finished vacuuming "postgres.public.pv_demo": index scans: 1
   ```
   
   The vacuum completes, but serially (`index scans: 1`, no parallel workers launched).
   
   ## Root cause
   
   Per the in-tree comment, parallel vacuum was left disabled during the PG14→PG16 merge because of unresolved **lock issues** in the MPP context (parallel workers inside a QE backend vs. the segment-local locking / dispatch model). The parallel path was `#if 0`'d rather than adapted.
   
   ## Proposed work
   
   1. Resolve the lock issues that block backend-parallel index vacuuming inside a segment QE.
   2. Re-enable the `#if 0`'d parallel path (`parallel_vacuum_init()` etc.) so `VACUUM (PARALLEL n)` and autovacuum can use parallel workers per segment.
   3. If it cannot be supported in the MPP model, replace the silent runtime downgrade with clear documentation and, ideally, a one-time notice rather than a per-relation `WARNING`.
   
   ## Notes
   
   - `max_parallel_maintenance_workers` currently defaults to `2` but has no effect on VACUUM because of the guard above.
   - Scope: heap tables with 2+ indexes. AO/AOCO tables use a different vacuum path and are out of scope for this specific guard.
   


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